Just saw this post. Not sure how i missed it.
A couple tips and information
1: avoid springfields under 800,000 and rock islands under 285,507. This is where the heat treatment failure was found.
2: scant stocks were a rebuild and will only be found as an original rebuild on 1903 versions. They will not fit an 03a3 and were not used except on existing 03a3 springfields and they will be marked with a small s. C stocks were used only on 03a4 versions when new and when found on another 03 version will have been done during a rebuild.
3: there is no need to avoid r marked parts, even on a smith corona. Most 03 rifles would have seen service of some sort during ww1 and into ww2 especially 1903 versions. The r mark only indicates it is a remington part. Most 03s unless you are lucky will be mix masters as they were being reordinanced for issue not collecting. This also happened at times because when troops were cleaning many times parts were all piled into a solvent bin for soaking. The 03 is very versatile this part fits every other 03 rifle part. No need to worry UNLESS you are buying one stated to be all original matching then parts should not differ. However smith corona used a lot of remington parts when building rifles for the military. Remember they were a typewriter company pressed into wartime service not an arms manufacturer originally
A rifle which has ping marks on the receiver near the serial is most likely a marine rifle as that is how they marked them as completed in reordinance. I am unaware of any indicator marks used by the army and have never come across any. Also most marine rifles will have a hole left side of chamber called a hatcher hole this was for venting gases out the side. 1903 rifles over the heat numbers are safe with modern ammunition baring any maintenance problems or miscare by previous owners. Modern ammunition is not significantly different in pressure from standard military 06 issue of the day. I would shoot hot loads but i dont shoot hot loads out of anything.
4: as mentioned national ordinance and such are post war civilian builds. They are generally safe to shoot but bear little if any collector value. Simply put they are a nice hunting or shooting rifle. Postwar there was a glut of parts due to the transition from 03 style to garands. The parts were obtained by 3rd party vendors who put them together for sales. Most are decent but of no interest to my collection.
5: as far as markings, i would hesitate to give advice on markings sight unseen. The stamps used have been heavily forged and you can actually buy very good copies online which are very difficult to detect in photos.
Welcome to the wonderful world of the 03. Let me know if i can help